On the subduction of the Apulian lithospheric microplate under the Euro-Asian one and the mantle wedge thermal convection as a possible mechanism of hydrocarbons upward transport in the Pannonia and the Vardar basins

UDC 530.311
Publication date: 02.10.2019
International Journal of Professional Science №10-2019

On the subduction of the Apulian lithospheric microplate under the Euro-Asian one and the mantle wedge thermal convection as a possible mechanism of hydrocarbons upward transport in the Pannonia and the Vardar basins

Gavrilov Sergei Vladilenovich
Kharitonov Andrey Leonidovich

1. Doctor of physical and mathematical sciences, Main scientist of the laboratory 102, Schmidt Institute of Physics of the Earth of the Russian Academy of Sciences
2. Candidate of physical and mathematical sciences, Leading scientist of the Main magnetic field laboratory, Pushkov Institute of Terrestrial Magnetism, Ionosphere and Radio Waves Propagation of the Russian Academy of Sciences
Review article
Abstract: For the non-Newtonian mantle rheology case the 2D thermal viscous dissipation-driven thermal convection in the mantle wedge above the Apulian lithospheric microplate subducting under the Euro-Asian plate is modeled numerically. The effects of the 410 km and 660 km phase transitions are taken into account. Within the framework of the model constructed the horizontal extent of the 2D heat flux anomaly observed in the rear of the Dinarides mountain belt corresponds to subduction velocity of ~10 mm per year which is close to that observed with the help of geodetic means. In the case of non-Newtonian rheology the upwelling convective flow transporting heat to the Earth’s surface locates at the distance from the trench corresponding to the actually observed 2D heat flux anomaly, the velocity in the convective vortices being from ~10 mm per year to ~10 m per year for the water content in the mantle wedge from 0.3Х10– 1 to 3Х10– 1 weight percent respectively. The convection cell dimension is of the order of the horizontal scale of the heat flux anomaly observed in the Pannonia basin and Vardar zone. Upwelling mantle wedge convective flow is indicated to be able to provide the mantle wedge hydrocarbons transport to the Earth’s surface for the mantle wedge mantle content over 0.3Х10– 1 weight percent.
Keywords: mantle wedge 2D thermal convection, non-organic mantle hydrocarbons transport, subduction angle and velocity, mantle rheology, phase transitions in the mantle.

Introduction

According to [3], the subduction of the Apulian lithospheric microplate under the Dinarides, Pannonia basin and Vardar zone is sufficiently flat and during the last  ~ 45 Ma occurred at the angle of ~ 25°, which remained unchanged during this time interval. The  genesis of the Dinarides mountain belt (with the transversal horizontal extent of ~ 300 km) apparently is of the thrust and fold nature, associated with the former collision and subduction of the oceanic branches of the Neo-Tethis and Alpine-Tethis as the result of thrusting of the African plate under the Eastern and Western Europe during the last 55 – 35 Ma [3]. In [1] numerous papers are referred, containing contradictory estimates of the relative motions of the Apulian lithospheric microplate and Euro-Asian plates, made on the basis of seismic, geophysical and geodetic data. In fig. 3 in [Op. cit.] the velocity of subduction of the Apulian lithospheric microplate under the Euro-Asian one is seen to amount to ~5 – 8 mm×a-1 according to referred works, while in [1] this velocity is estimated to be of the order of ~5 mm×a–1 according to calculations based on geodetic observations. In [12] the Pannonia basin and Vardar zone are noted to be the zones of the Middle Miocene extension occurred ~14 – 11.6 Ma ago, which led to the lithosphere thinning, these zones being the back-arc basin characterized by the back-arc spreading. At that time the single mountain belt parallel to Apulian shore was split into Carpathians and Dinarides and the shallow Pannonian sea was formed, which existed approximately to 600 thousand years ago. Presently the Pannonian oil- and gas-bearing basin is situated in this region. Here the conditions are clarified under which the centre of the back-arc spreading initiates as the result of convective instability, driven by the dissipative heat release in the mantle wedge above the subducting Apulian lithospheric microplate.

According to [4, 8, 9] two types of dissipation-driven small-scale thermal convection in the mantle wedge are possible, viz. the 3D finger-like convective jets, raising to volcanic chain, and 2D transversal Karig vortices, aligned perpendicularly to subduction. These two types of convection are shown to be spatially separated due to the pressure and temperature dependence of mantle effective viscosity, the Karig vortices, if any of them formed, being located behind the volcanic arc [4]. There are contradictory judgments on the velocity of subduction of the Apulian lithospheric microplate under the Euro-Asian one, although the order of magnitude of the present-day subduction velocity  (~ 10 mm×a–1) can apparently be regarded as established sufficiently reliably. The mountainous massif Dinarides locates parallel to the north-eastern shore of the Adriatic sea, and probably is of the thrust-and-fold nature. The 2D maximum of the heat flux anomaly of ~100 mW×m–2 observed in the rear of the Dinarides massif in the Pannonia basin and the Vardar zone [10] can be assumed to owe its origin to the convective heat supply from the mantle wedge. Numerical modeling of the 2D convection, occurring in the mantle wedge in the form of the Karig vortices and presumably transporting heat upwards, allows to judge about the mean water content in the mantle wedge and to assume the mantle hydrocarbons to be transported to the Earth’s surface by the upwelling convective flows. Numerical convection models accounting for the pressure-, temperature- and stress-dependence of viscosity fit best to observational data in the case of non-Newtonian rheology at the mantle water content of ~ (0.3 – 3)´10-1 weight % for the velocity of subduction of ~ 10 mm×a-1 during the Middle Miocene. In [15] such rather a high water content (and even greater one, up to 3 weight %) can be observed in the mantle wedge in the mantle transition zone. The Middle Miocene subduction velocity of ~10 mm×a–1 during the formation of the Pannonia basin is of the order of the observed presently, or, can somewhat exceed it because of the gradual diminution of the velocity of convergence of African and Euro-Asian plates.

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